1. Field of the Invention
The present invention relates to a photovoltaic element, a producing method therefor, and a solar cell module. More particularly, the present invention relates to a photovoltaic element comprising a substrate having a through-hole formed therein through which electrodes arranged in the front and the back surfaces of the substrate are electrically connected, whereby a grid electrode is made unnecessary and the loss of incident light can be suppressed, a producing method therefore, and a solar cell module using the photovoltaic element. Further, the present invention relates to a photovoltaic element wherein a rectification layer having a rectification characteristic in the reverse direction to the voltage of the photo-electromotive force generated in a photoelectric conversion layer is formed to unit a bypass diode, to improve the processibility and lower the cost, a producing method therefore and a solar cell module using the photovoltaic element.
2. Related Background Art
Recently, in order to solve the problems of depletion of fossil fuel and the environmental problems induced by utilizing fossil fuel, solar energy has drawn attention as clean energy for the next generation.
Many photovoltaic elements used today comprise electrodes formed on a light-receiving face side and a non-light-receiving face side of a photoelectric conversion layer, a transparent electrode transmitting light formed on the light-receiving face side of the photoelectric conversion layer, and a metal electrode formed on the non-light-receiving face side of the photoelectric conversion layer.
The transparent electrode transmitting the light has a high electric resistance as compared with the metal electrode and causes an electric power loss at the time when an electric current flows in the transparent electrode. Hence, a grid electrode is used together for the purpose of lowering the electric power loss due to the transparent electrode. However, the grid electrode shuts out a part of sunlight incident upon the photoelectric conversion layer. Therefore, there occurs another problem that the electric power loss is caused by the grid electrode.
Therefore, many research and investigation institutes have been performing investigation and development of photovoltaic elements capable of suppressing an electric power loss caused by the transparent electrode to the minimum level and making it possible to omit the grid electrode.
Japanese Patent Application Laid-Open No. 6-151914 discloses a photovoltaic element having a structure in which a hole is formed in a photoelectric conversion layer with excimer laser and a light-receiving face electrode film formed on the light-receiving face side of the photoelectric conversion layer and a back electrode film formed in the non-light-receiving face side are electrically connected through the hole. The electric charge generated in the light-receiving face of the photoelectric conversion layer flows from the light-receiving face electrode film to the back electrode film through the hole, so that a grid electrode can be omitted, and the photovoltaic element has an advantage that the electric power loss attributed to the shadow of the grid electrode can be removed.
Further, in order to further simplify the production of the photovoltaic element in which a grid electrode is omitted, there are many ideas proposed relevant to the producing method in which the hole is formed in the initial stage of the production process and after that thin films are successively stacked, and the like.
Japanese Patent Application Laid-Open No. 8-64850 discloses a method of producing a photovoltaic element by successively forming an insulating layer on one side of a metal substrate having through-holes at constant intervals, a photoelectric conversion layer and further a transparent electrode layer on the other side, and a back electrode layer on the one side. Also, Japanese Patent Application Laid-Open No. 11-261086 proposes a method of producing a photovoltaic element by forming an insulating layer and a back electrode layer on a conductive substrate and then forming through-holes or recessed portions, and further successively stacking a photoelectric conversion layer and a transparent electrode layer.
The respective semiconductor layers of a photovoltaic element are generally formed by chemical vapor deposition, a sputtering method and the like in vapor phase or vacuum. In the case of forming the layers by these methods, in order to prevent a thin film composition adhered to the inner walls of a film formation apparatus from dropping on the processing face of a substrate or the like, it is preferable to carry out film formation in the state of facing downward the face of the substrate on which a layer is to be deposited.
However, in the method of producing a photovoltaic element disclosed in Japanese Patent Application Laid-Open No. 8-64850, layers are formed in the order on a non-light-receiving face side, a light-receiving face side, and a non-light-receiving face side of a substrate. For that, when film formation is carried out in the state of facing downward the face of the substrate on which a layer is to be deposited, the production process is divided into a first step of forming an insulating layer on the non-light-receiving face side of the substrate, a second step of forming a photoelectric conversion layer and a transparent electrode layer on the light-receiving face side of the substrate, and a third step of forming a back electrode layer on the non-light-receiving face side of the substrate. As a result, apparatuses are required to be made ready for respective steps. Like this, division of the process of forming the layers on the non-light-receiving face side of the substrate into two steps requires a long time necessary for production of the photovoltaic element, an increased number of apparatuses, and increased labor of workers. Also the cost of the photovoltaic element is elevated.
On the other hand, the method of producing the photovoltaic element proposed in Japanese Patent Application Laid-Open No. 11-261086 comprises forming through-holes after an insulating layer and a back electrode layer are stacked on a conductive substrate. For that, when the perforation process is carried out by a laser processing method or a mechanical processing method, it is required to prevent melting of a metal by heat and electric communication of the conductive substrate and the back electrode owing to the burr generated by the mechanical force.
Further, since the output voltage is generally insufficient in case of only one photovoltaic element, a plurality of photovoltaic elements connected in series are used. However, in case of connecting a plurality of photovoltaic elements in series, the following problems occur.
In the case some of elements become impossible to generate electric power owing to that the sun rays are shut by shadows of buildings, snow fall or the like, the whole generated voltage from normally working other elements is applied as inverse voltage to the elements which do not generate electric power. If the inverse voltage exceeds the withstand voltage of the elements, the probability of breakdown of the elements is elevated. In order to avoid that, bypass diodes are connected in parallel in the inverse direction for the respective photovoltaic elements connected in series. Japanese Patent Application Laid-Open No. 7-302923 proposes photovoltaic elements connected with bypass diodes having lead wires made of a metal foil material.
However, the photovoltaic elements disclosed in Japanese Patent Application Laid-Open No. 7-302923 require a work of connecting bypass diodes to an electrode of each one of the photovoltaic elements and thus the work is complicated and mass production in-a-line is made difficult.
Further, if bypass diodes are attached after formation of the respective layers constituting the photovoltaic elements, the flatness of the parts where the bypass diodes are formed is deteriorated. As a result, at the time of performing any processing on the photovoltaic elements, the stress is applied concentrically upon the bypass diodes and the bypass diodes are possibly broken.
An object of the present invention is to solve the above described problems respectively or as a whole, and to provide a through-hole type photovoltaic element with excellent productivity and production yield; a producing method therefore, and a solar cell module using the photovoltaic element.
For such object, the present invention provides a photovoltaic element comprising a conductive substrate having a through-hole, a transparent electrode layer and a photoelectric conversion layer in this order from a light-receiving face side on the major surface of the light-receiving face side of the conductive substrate, an insulating layer and a back electrode layer in this order from the light-receiving face side on the major surface of the non-light-receiving face side of the conductive substrate; wherein the foregoing photoelectric conversion layer and the foregoing transparent electrode layer are formed continuously from the major surface on the light-receiving face side to at least a part of the inner wall face of the foregoing through-hole; the foregoing insulating layer and the foregoing back electrode layer are formed continuously from the major surface of the non-light-receiving face side to at least a part of the inner wall face of the foregoing through-hole; the foregoing photoelectric conversion layer, the foregoing transparent electrode layer, the foregoing insulating layer, and the foregoing back electrode layer are successively stacked in this order in at least a part of the inner wall face of the foregoing through-hole; and the foregoing transparent electrode layer and the foregoing back electrode layer are electrically communicated through the foregoing through-hole.
Further, the present invention provides a method of producing a photovoltaic element comprising a conductive substrate having a through-hole, a transparent electrode layer and a photoelectric conversion layer in this order from a light-receiving face side on the major surface of the light-receiving face side of the conductive substrate, and an insulating layer and a back electrode layer in this order from the light-receiving face side on the major surface of the non-light-receiving face side of the conductive substrate; wherein the method comprises a first step of successively forming the photoelectric conversion layer and the transparent electrode on the major surface of the light-receiving face side of the foregoing conductive substrate and on the inner wall face of the through-hole and a second step of successively forming the insulating layer and the back electrode layer on the major surface of the non-light-receiving face side of the foregoing conductive substrate and on the inner wall face of the through-hole.
According to the photovoltaic element of the present invention, since the electric current generated in the photoelectric-conversion layer is led to the back electrode layer from the transparent electrode layer through the through-hole or to the transparent electrode layer from the back electrode layer, no grid electrode is required to be formed on the light-receiving face of the photovoltaic element. Consequently, the loss owing to the shadow of the grid electrode itself can be eliminated.
Further according to the producing method of the photovoltaic element of the present invention, the photoelectric conversion layer, the transparent electrode layer, the insulating layer, the back electrode layer are formed in this order on the conductive substrate having the through-hole, whereby a plurality of layers such as semiconductor layers to be formed on one face of the substrate can be formed in a step one time. Since the respective layers can efficiently be formed, the productivity is high.
That is, the photoelectric conversion layer and the transparent electrode layer are formed on the light-receiving face side of the conductive substrate and then the insulating layer and the back electrode layer are formed in the non-light-receiving face, so that the semiconductor layers and the like to be formed on one side of a substrate can be formed in one step. Consequently, the number of the steps can be lessened.
Further, another object of the present invention is to provide a through-hole type photovoltaic element integrated with bypass diodes excellent in productivity and processibility, a producing method therefor, and a solar cell modules using it.
For such object, the present invention provides a photovoltaic element comprising a conductive substrate having a through-hole, a transparent electrode layer and a photoelectric conversion layer in this order from a light-receiving face side on the major surface of the light-receiving face side of the conductive substrate, wherein the photovoltaic element further comprises a rectification layer and a back electrode layer in this order from the light-receiving face side on the major surface of the non-light-receiving face side of the conductive substrate; the foregoing photoelectric conversion layer and the foregoing transparent electrode layer are formed continuously from the major surface on the light-receiving face side to at least a part of the inner wall face of the foregoing through-hole; the foregoing rectification layer and the foregoing back electrode layer are formed continuously from the major surface of the non-light-receiving face side to at least a part of the inner wall face of the foregoing through-hole; the foregoing transparent electrode layer and the foregoing back electrode layer are electrically communicated with each other through the through-hole; and the rectification characteristic of the foregoing rectification layer is in the inverse direction to the voltage of the photoelectric motive force of the foregoing photoelectric conversion layer.
Further, the present invention provides a method of producing a photovoltaic element comprising a conductive substrate having a through-hole, a transparent electrode layer and a photoelectric conversion layer in this order from the light-receiving face side on the major surface of the light-receiving face side of the conductive substrate, and a rectification layer and a back electrode layer in this order from the light-receiving face side on the major surface of the non-light-receiving face side of the conductive substrate; wherein the method comprises a first step of successively forming the photoelectric conversion layer and the transparent electrode on the major surface of the light-receiving face side of the foregoing conductive substrate and on the inner wall face of the through-hole and a second step of successively forming the rectification layer and the back electrode layer on the major surface of the non-light-receiving face side of the foregoing conductive substrate and on the inner wall face of the through-hole.
According to the photovoltaic element of the present invention, since the rectification layer formed on the major surface of the non-light-receiving face side of the conductive substrate has the rectification characteristic in the inverse direction to the voltage of the photoelectric motive force of the photoelectric conversion layer, the layer performs a function as a bypass diode. Consequently, at the time when the voltage in the inverse direction is applied to the photovoltaic element, the electric current generated flows through the rectification layer, so that breakdown of the photoelectric conversion layer owing to the voltage in the inverse direction can be prevented.
Also, since the bypass diode functioning as the rectification layer is integrated with the photovoltaic element, no work for attaching bypass diodes one by one to the photovoltaic element in a post step is made unnecessary and consequently the productivity is improved. Further, since the bypass diode is integrated as the rectification layer, the flatness of the photovoltaic element is improved and the photovoltaic element can easily be processed.
Further, the present invention provides a solar cell module comprising photovoltaic elements described above wherein at least a part of the photovoltaic element is coated with a sealing material. Consequently, each photovoltaic element is provided with heat resistance, weathering resistance, insulation breakdown resistance, and the like, thereby resulting in improvement of the reliability of each photovoltaic element.
Other characteristics and effects of the present invention will be described in details later with reference to the drawings.